The use of fluidized beds has been recognised as an efficient means of generating heat. In these, air is passed through a perforated air distribution plate to fluidize the bed material and promote the combustion of the fuel. The heat produced by the combustion can be used to produce hot water or steam by means of heat transfer tubes, through which a heat exchange medium, such as water, flows. In a fast fluidized bed process air is forced through the air distributor plate and through the fluidized bed at a high velocity to cause fine particle materials to be entrained by the air, whereby a particle suspension is formed, the solid particle content of which continuously decreases from the distributor plate to the upper part of the reactor. The fast fluidized bed process offers a high heat transfer coefficient between the bed material and the heat transfer surfaces and achieves a reduction in boiler size compared with the use of a conventional fluidized bed.
A fluidized bed reactor is capable of burning fuels of different calorific values, but if different fuels have to be burned in the same reactor, then difficulties in maintaining the appropriate combustion temperature may arise. The combustor and its heat transfer surfaces will in this case be designed for the fuel of the lowest calorific value. Changing over from a low grade fuel to a high calorific fuel means that temperature in the combustor may rise to a point, where sintering of the bed material occurs. To avoid too high a temperature an air excess may be used to controll the combustion temperature. Another way may be to inject water to control the temperature. Both methods mean, however, heat losses and a decrease of the boiler efficiency.
The combustion of different fuels in the same combustor means also difficulties in designing the superheaters for a steam generating system. The location of the superheater is normally after the hot cyclone for low grade fuels. When burning a high grade fuel at the same capacity the flue gases have too low a temperature for sufficient superheating.
When burning high sufur coal the control of the combustion temperature is of the outmost importance to obtain an optimal absorption of SO.sub.2 in the temperature range of 800.degree.-950.degree. C. utilizing limestone and/or dolomite as an absorbing agent. Combustion of high sulfur fuels causes also corrosion (sulfidation) of heat transfer surfaces. In this case a protection of the heat transfer surfaces from corrosive and sometimes also erosive conditions is of importance. Especially when the superheater is located in the bed zone.
The start-up of the combustor with heat transfer surfaces in the lower part may causes problems due to inefficient heat transfer from a over-bed burner. In this case inactivated heat transfer surface will facilitate the start-up procedure.